Close associations between adult males and lactating females and their dependent infants are not commonly described in non-monogamous mammals. However, such associations [called ‘friendships’; Smuts, 1985] are regularly observed in several primates in which females mate with multiple males during the fertile period. The absence of mating exclusivity among ‘friends’ suggests males should invest little in infant care, raising questions about the adaptive significance of friendship bonds. We evaluated the extent to which friendships in four multi-male, multi-female baboon (Papio Cynocephalus) groups in Amboseli, Kenya represent joint parental care of offspring or male mating effort. We found evidence that mothers and infants benefited directly from friendships; friendships provided mother-infant dyads protection from harassment from adult and immature females. In addition, nearly half of all male friends were the genetic fathers of offspring and had been observed mating with mothers during the days of most likely conception for those offspring. In contrast, nearly all friends who were not fathers were also not observed to consort with the mother during the days of most likely conception, suggesting that friendships with non-fathers did not result from paternity confusion. Finally, we found no evidence that prior friendship increased a male’s chances of mating with a female in future reproductive cycles. Our results suggest that, for many male-female pairs at Amboseli, friendships represented a form of biparental care of offspring.